It is known to separate particles, such as dirt and dust particles, from a fluid flow using mechanical filters, such as mesh and foam filters, cyclonic separators and electrostatic separators.
Known separating apparatus include those used in vacuum cleaners, for example cyclonic separating apparatus. Such cyclonic separating apparatus are known to comprise a low efficiency cyclone for separating relatively large particles and a high efficiency cyclone located downstream of the low efficiency cyclone for separating the fine particles which remain entrained within the airflow (see, for example, EP0042723B).
Known electrostatic filters include frictional electrostatic filters and electret medium filters. Examples of such filters are described in EP0815788, U.S. Pat. No. 7,179,314 and U.S. Pat. No. 6,482,252.
Such electrostatic filters are relatively cheap to produce but suffer from the disadvantage that their charge dissipates over time resulting in a reduction of their electrostatic properties. This in turn reduces the amount of dust the electrostatic filter can collect which may shorten the life of both the electrostatic filter itself and any further downstream filters.
Known electrostatic filters also include filters where dust particles in an airstream are charged in some way and then passed over or around a charged collector electrode for collection. An example of such an electrostatic filter is described in JP2007296305 where dust particles in an airstream are charged as they pass a “corona discharge” wire and are then trapped on an electrically conductive filter medium located downstream of the corona discharge wire. A disadvantage with this arrangement is that they are relatively inefficient, are made from relatively expensive materials and the collector electrodes require constant maintenance in order to keep them free of collected dust. Once the collector electrodes are coated in a layer of dust they are much less efficient.
Another example of an electrostatic filter is shown in GB2418163 where the dust particles in an airstream are charged as they pass a corona discharge wire located inside a cyclone. The charged dust particles are then trapped on the walls of the cyclone which are coated in a conductive paint. While this arrangement is compact it suffers from the disadvantage that dust collects on the inside of the cyclones. Not only does this require constant and difficult maintenance removing dust from the walls of the cyclone, but also any dust trapped inside the cyclone will interfere with the cyclonic airflow decreasing the separation efficiency of the cyclone itself.